This invention relates to a method of forming a composite metal-ceramic helical structure for use in a Travelling Wave Tube (TWT) amplifier, the composite structure having good thermal conductivity and superior electrical characteristics over composite helix structures formed by conventional methods.
In a broader sense this invention also relates to a method for joining concentric cylinders and, in particular, to a method of joining an inner cylinder of relatively thin ductile metal to an outer cylinder of relatively stiff ceramic material. Thus, without restriction to the particular processes described herein for the purposes of disclosing preferred embodiments and best modes presently known for carrying out the invention, this invention relates to both a process for forming very precisely configured composite metal-ceramic helical structures and a process for forming a high strength hermetic bond with good thermal conductivity between an inner cylinder made of a ductile metal material such as copper and an outer ceramic cylinder.
TWT's are generally a type of high power, very high frequency signal amplifier providing, for example, a continuous wave signal output in the one kilowatt power range at frequencies between eight and eighteen gigahertz. An essential element common to most TWT's is a conducting metal helical structure which carries an input signal and modulates an electron beam traveling down the central bore of the helix. Physically this helix may be very small, for example, less than one-tenth of an inch in diameter and slightly over two inches in length. Generally the helix operates at relatively high temperatures on the order of five hundred degrees centigrade. Because of their small size and high operating temperatures, heat dissipation is a critical factor in the design of helix structures.
TWT helices have been conventionally configured as an all-metal helical structure, such as copper, supported on several sides by a number of non-conducting dielectric rods extending along the length of the metal helix and attached to the outside of this metal structure. While rod supported all-metal helices usually provide satisfactory electrical characteristics (e.g. low RF signal attenuation and acceptable dielectric loading), these configurations typically have poor thermal characteristics. The minimal contact between the turns of the metal helix and the dielectric rods provides only a limited path for heat dissipation, with the result that portions of each turn making up the metal helix may develop localized hot spots at temperatures high enough to melt the metal.
Another TWT helix configuration which has been attempted in the past is a composite metal-ceramic structure in which a ceramic helix is bonded to a metal helix along the entire outer surface of the metal structure. This composite metal-ceramic helix type configuration is obtained by first wrapping a metal ribbon about a mandrel to form the metal helix, wrapping one or more covering tapes about the metal helix so as to bridge the gaps between each of the turns of the metal helix, and then depositing a ceramic compound onto the exposed outer surface of the metal helix by sputtering or plasma deposition techniques. Excess ceramic and the spacer tapes are then removed by etching. Thereafter, the outer surface of the deposited ceramic is machined to obtain requisite outer dimensions and tolerances. Composite TWT helical structures formed by ceramic deposition processes typically possess good thermal transfer properties but also display a number of unacceptably poor electrical characteristics such as, for example, very high RF losses. In addition, the fabrication process is very difficult and expensive to implement. It is typically difficult to accurately register the pitch of the covering tapes with the pitch of the metal helix and some of the final processing steps such as the etching and machining steps are difficult to properly carry out.
Thus there exists a need for a method of producing metal-composite helices for TWT applications which is simpler and less expensive to practice, and which provides a resulting metal-ceramic helix structure having improved electrical characteristics. As discussed more fully in the Summary and Detailed Description below, Applicants have achieved a novel and unique process for forming composite metal-ceramic helix for TWT applications by first concentrically joining a metal cylinder to a ceramic cylinder and then machining the joined cylinders into a helical structure.
Various attempts have been made to develop a satisfactory process which will bond a metallic material such as copper to the inner surface of a non-metallic material such as a ceramic, carbon or diamond to form a high strength bond with good thermal conductivity which is free from any air-pockets or occlusions in the area between the metal and the non-metallic material. One method attempted in the prior art is disclosed in U.S. Pat. No. 2,570,248 "METHOD OF METALIZING AND BONDING NON-METALLIC BODIES" wherein a process for joining metallic bodies to non-metallic refractory bodies utilizes a mixture including titanium hydride and a solder metal such as copper, silver, gold, or the like applied to the body to be metalized or bonded. The hydride is thereafter disassociated by the application of heat in the presence of a solder metal. This heating is preferably done in a non-oxidizing atmosphere, such as pure dry hydrogen. The coated body is heated in the inert medium to a temperature sufficient to disassociate the hydride and melt the metallic material. In another embodiment, the non-metallic part and the metal part are placed in close contact, as in a common brazing operation, and a hydride mixture, such as titanium hydride and a copper powder, is placed at or adjacent the junction of the two bodies in a pure dry hydrogen atmosphere until the hydride is disassociated and a melting of the titanium and copper results. While such a process may be satisfactory in certain instances, pockets or occlusions may occur between the metallizing or metallic material and the non-metallic body, particularly when cylindrical bodies are concentrically joined.
Another attempt to resolve this problem is disclosed in U.S. Pat. No. 2,779,279 "APPARATUS FOR SECURING A TUBE OR TUBES IN A BODY MEMBER" in which a grooved body member has a tube inserted therein. A predetermined explosive charge is placed within the tube in a position adjacent to the grooved body member and a pair of explosion confiners, in the form of spheres, are placed at either end of the explosive charge. Upon ignition of the explosive charge, the blast is directed by a momentary confinement, due to the joining together of the spheres, which prevents them from moving away from each other. In this manner the explosion expands the tube to conform to the grooves in the body member. While such a system may be satisfactory for some purposes, the conductivity of the metallic material may be non-uniform due to the inherent nature of the explosion and residual materials from the explosion may or may not remain within the confines of the body member. Thus there still exists a need for a method of concentrically joining a metal cylinder within a non-metallic or ceramic cylinder which is suitable for TWT applications.